The invention relates to an acousto optical element, in particular for disposition in the beam path of an optical device, preferably a microscope, comprising a crystal for transmitting light, wherein via an electrical converter sound waves are generated and are passed through the crystal and wherein the sound waves passing through the crystal change the optical properties of the crystal and therefore influence the light penetrating through the crystal.
The present invention relates in general to an acousto optical element, i.e. to an element by means of which light beams can be manipulated depending on the acoustic situation.
Acousto optical elements or components are known from practical application in many varieties. In this regard, it is particularly referred to acousto optical elements such as an AOM (acousto optical modulator), an AOD (acousto optical deflector), an AOTF (acousto optical tunable filter), frequency shifter or the like. For related embodiments comprehensive scientific literature is known, for instance “Acousto-Optics” from Adrian Korpel, New York 1997, “Acousto-optic devices” from Jieping Xu and Robert Stroud, New York 1992 and “Design and Fabrication of the Acousto-Optic devices” from Akis P. Goutzoulis and Dennis R. Pape, New York 1994.
In this connection, it is pointed out that with regard to optoacoustic elements there are various terms, explanations and operating principles and properties. In this respect it seems necessary to discuss the invention behind this background, namely discuss general properties of optical components.
Acousto optical elements consist typically of a so-called acousto optical crystal and an electrical converter attached thereto (in the literature often named transducer). Typically, the converter comprises a piezo material as well as one electrode above and one electrode below the piezo material. By electrically subjecting the two electrodes to a radio frequency that is typically in the range of 30 to 800 Mhz the piezo material is induced to oscillate so that a sound wave, i.e. an acoustic wave is generated and passes thereafter through the crystal. Mostly, the acoustic wave is absorbed or reflected at the opposite side of the crystal after it has passed through an area of optical interaction.
Acousto optical crystals have the properties that the generated wave changes the optical properties of the crystal, wherein by means of the acoustic wave sort of an optical grating or a comparable, optically active structure is induced, for example a hologram. By means of the crystal the light passing therethrough is deflected at the optical grating. Accordingly, the light is deflected in several diffractive orders in the direction of diffraction.
Acousto optical elements exist that influence the entire incident light more or less independently from the wavelength. In this connection, as examples, the aforementioned elements AOM, AOD and frequency shifters are pointed out. Further, meanwhile elements exist that act upon incident light selectively with regard to individual wavelengths (AOTFs). Frequently, such acousto optical elements consist of birefringent crystals, such as tellurium dioxide, wherein the position of the crystal axis in relation to the plane of incidence of the light and its polarization determine the optical properties of the respective elements.
For practical application, use of the light that has not been influenced by the diffraction (zeroth diffractive order), the light that has been deflected into the various diffractive orders, or both types of light are used.
In case of a conventional acousto optical element as it is known from practical application, the radio frequency is supplied through a coax cable to the acousto optical element. There, on a circuit board, an impedance matching is conducted, so that a radio frequency reflection is prevented and therefore as high a radio frequency power as possible arrives at the crystal that has a different impedance as the radio frequency cable. From the circuit board the radio frequency is transmitted further to the converter (transducer) that is provided on the crystal where the acoustic wave is generated.
It is essential for this acousto element according to prior art that various layers are provided on the surface of the crystal. The lowermost layer that is applied to the crystal consists of metal and provides the negative electrode. It is typically connected via a circuit board with the outer conductor of a coax cable transmitting the radio frequency. Upon the lowermost metal layer, a layer of piezo material follows. This material is typically lithium niobate. Upon this layer of piezo material the second metallic electrode follows, typically connected to the inner conductor of the coax cable. When subjecting the two electrodes to the radio frequency an alternating electrical field is generated between the two electrodes that induces the piezo element to oscillate between them so that from this area an acoustic wave spreads through the interior of the crystal. The acoustic wave is typically reflected away from the opposite side of the crystal. By roughening the opposite side of the crystal it is possible to absorb a major portion of the acoustic wave so that as little reflection as possible back into the acousto optical interaction area of the crystal occurs.
In particular when the converter is large the capacitance of the capacitor formed by the two electrodes is much too high for allowing a useful impedance matching between the radio frequency cable and the acousto optical crystal. In this situation a trick is applied, namely the converter is split into two parts, wherein the two parts are connected in series. For this purpose it is necessary to pole the piezo material in both parts differently so that the two converter halves do not work against each other. In that case the inner conductor of the coax cable is connected to the positive electrode. The outer conductor of the coax cable is connected with the negative electrode that is provided on the other side of the piezo material. The converter provides a capacitance to which the alternating electrical field is applied.
In case of a split converter the inner conductor of the coax cable is connected with the positive electrode that, however, does at maximum cover half of the front side of the converter. The respective counter electrode is at the same time the counter electrode with respect to the negative electrode and covers the other half of the front side of the converter. In this case, the capacitances of two capacitances that are electrically interconnected in series are provided. Consequently, by splitting the converter into two parts, a reduction of the capacitance down to one quarter is achieved wherein the size of the sound field generating area is maintained. In this connection it has to be taken care that the piezo layers of the two halves of the converter are poled in opposite directions since these would otherwise work against each other and the generated sound field would then not be the same as in case of the non-split converter.
The structure of the converter, in particular its size and shape, determines how the acoustic field in the interior of the crystal is shaped. This results in defining the optical properties of the electrically interconnected crystal.
In case of an acousto optical tunable filter (AOTF) the spectral characteristics of the deflected light is determined substantially by the Fourier-transformed of the converter form. This means that wide converters provide a sharper spectral transmission function in comparison with more narrow converters. Consequently, an AOTF with a wide converter cuts out from the incident light a spectrally more narrow range compared to an AOTF with a converter that is narrower. The AOTF with a wide converter comprises therefore a higher spectral resolution. In the reverse case, an AOTF with a narrower converter has a higher spectral bandwidth and deflects therefore more portions of the spectrum of the incident light.
While in the past acousto optical elements as known from the prior art, in particular AOTF, were typically used for adjusting and controlling the light intensities, there has been a meanwhile stronger tendency making it necessary to use such acousto optical elements for cutting or filtering out a particular portion from the light having more or less broad band spectrum. In this connection it is for instance referred to DE 101 15 488 A1.
Such an application of the acousto optical element can be mainly found in cutting out particular spectral portions of light from a light source emitting a continuous spectrum or a spectrum of a wide bandwidth for the purpose of illumination (white light laser, broad band laser, ultra-short pulsed laser, superluminescent-LED, or other superluminescent light sources, ASE light sources, light bulbs, Point-Source-LEDs and other LEDs, sunlight, starlight or the like). Another field of practical application is cutting out particular light portions of a spectrum for the purpose of detection. In this connection, for example, it is referred to conventional, programmable spectral filters. Moreover, the application of an acousto optical element as a component in a programmable beam splitter is of significance in this connection.
However, for the acousto optical elements that are available at present and in particular AOTFs the optical properties of the respective elements are predefined and non-changeable, namely by the choice of a particular converter design. A later change or adjustment of the bandwidth of the spectrum of an AOTF is so far not possible while, in light of the explanations above, this would be desirable.